So You Want to Be a Wizard by Diane Duane
I read this book quite a few years ago and recently have been catching up with some of the other titles in the Wizard series. Premise of this first book in the series is that Nita - bookworm and science geek - takes refuge in the children's department of public library to escape a group of bullies. While there, she spots a book entitled "So You Want to Be A Wizard" and as she reads, realizes that she posesses many of the characteristics needed to train to become a wizard. So begins this series, where wizards work to slow entropy in the universe by fighting against dark forces.
Nita is a well-drawn character who over the course of the series matures, as she encounters challenges and even tragedies in both the real and fantasy world. Despite the premise of young adults training to become wizards, the series is very different from the Harry Potter Books. The magic is rooted in science, particularly physics, and the wizards must always be careful to balance their use of magic against the energy available in the universe. A strong series and highly recommended